Along with energy crisis, many people have been looking for clean energy for eco-friendly development. Semiconductor-based photocatalysts are one of the most attractive candidates for photosensitized decomposition of water capable of producing hydrogen during a process. Among the photocatalysts, TiO2 having relatively high reactivity and chemical stability under ultraviolet irradiation has received attention. TiO2 nanoparticles have a large surface area for a high speed of a surface reaction and thus have been widely used as a photocatalyst for various solar-based clean energy and environmental technologies.
In order to maximize absorption of TiO2 under the sunlight, many researchers have already tried to modify the composition of TiO2 by performing additional doping with metals, inorganic substances, and Ti3+. The light absorbing property of TiO2 has been improved by doping. By way of example, nitrogen-doped TiO2 reacts upon irradiation of the sunlight, but still insufficiently absorbs visible light and infrared light.
Conventionally, many researches tried to reduce a band gap of TiO2 with respect to the sunlight and increase absorption of the sunlight by using various methods and reagents.
Meanwhile, in 2011, X. B Chen et. al. first reduced TiO2 whose surface morphology was modified with a hydrogen gas, and formed a disordered layer on a surface of TiO2 [Xiaobo Chen, Lei Liu, Peter Y. Yu and Samuel S. Mao, “Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals” Science, vol. 331, no. 6018 (Feb. 11, 2011), pp. 746-750]. Light absorption and charge separation was also increased. Various conditions for TiO2 treated with hydrogen according to the present method have been disclosed in numerous articles and publications. However, annealing with a H2 gas at an ultra-high temperature is very dangerous in industrial production. Accordingly, a novel preparing method as an alternative to the conventional technology needs to be searched.